The basic unit of time in UMTS radio signals is a 10 milli-second (ms) radio frame, which is divided into 15 slots of 2560 chips each. UMTS radio signals from a cell (or base station) to a UMTS receiver are “downlink signals,” while radio signals in the reverse direction are termed “uplink signals.”
The physical layer of the universal mobile telecommunication system (UMTS) wideband code-division multiple access (WCDMA) standard uses direct sequence spread spectrum (DSSS) modulation with a chip rate of 3.84 Mcps. The frequency division duplex (FDD) mode carries the uplink and the downlink channels on separate frequency bands of 5 MHz each. This mode is typically used for large outdoor cells because it can support a larger number of users than time division duplex (TDD) mode. In TDD mode, the transmissions share the same uplink and downlink channels during different time slots. The TDD mode does not support as many users as the FDD mode, and hence, TDD mode is more suitable for smaller cells. TDD mode is also more suited for carrying asymmetric traffic compared to FDD mode.
An important procedure performed by a receiver within a UMTS network, for example a CDMA mobile receiver, is the cell search operation. Cell searching typically is performed by a cell search system that is incorporated as part of the receiver. The cell search system is activated after the receiver is powered on to determine synchronization information pertaining to the cell in which the receiver is located. The cell search operation is a three-stage process. That is, the cell search system performs slot synchronization (primary synchronization), frame synchronization and scrambling code group determination (secondary synchronization), and scrambling code determination.
After power-up, the mobile terminal (MT) has to perform several operations before voice/data communications can begin. First, the receiver needs to implement automatic gain control (AGC) in order to scale the received signal power and prevent clipping at the analog-to-digital converter. This process first can be performed on the synchronization channel (SCH) and later the descrambled common pilot channel (CPICH) can be used once the cell's scrambling code is acquired.
Next the receiver needs to acquire timing synchronization. Timing synchronization can be achieved from the SCH channel. The MT searches for the strongest SCH signal that it can find and that signal determines with which cell the MT will initiate communications. Since the SCH channel is periodic, the receiver can correlate against the primary SCH to derive a timing error. Based on this channel, the receiver can achieve chip, symbol and slot synchronization.
The primary SCH carries the same signal for all cells in the system. The secondary SCH is different for each cell and carries a pattern of secondary synchronization codes (SSCs) that repeat every frame. Once the MT receives this sequence, it will have frame synchronization.
In performing cell searching, the cell search system accesses a synchronization channel (SCH) and a common pilot channel (CPICH) of the received wireless signal. The SCH is a composite channel formed from a primary SCH and a secondary SCH. Within each slot, the primary SCH specifies a primary synchronization code (PSC). The primary SCH, however, only contains data during the first 256 chips of each 2560 chip slot. As is known, “chip” or “chip rate” refers to the rate of the spreading code within a CDMA communication system.
In addition, the pattern identifies which scrambling code group the current cell's scrambling code belongs to. There are 64 scrambling code groups and each group contains eight scrambling codes. Once the MT has determined the current cell's scrambling code group, the search for the current cell's scrambling code is narrowed to the eight codes in that group.
The typical acquisition process for a carrier based receiver is as follows:
1. Primary Cell Search
2. Secondary Cell Search
3. Scrambling Code Determination
4. Multipath Searching
5. Finger Assignment
6. Locking of Code Tracking and Automatic Frequency Control (AFC) loops
7. Maximal Ratio Combining (MRC) of finger output
8. Receiver lock is acquired and data can be sent to upper layers
This acquisition process is long and involved and can take on the order of several seconds to complete. This waiting period is annoying for the cell phone/mobile station/mobile device user when he/she turns on his/her phone and a method for shortening the acquisition process is clearly desirable.
EP1179962 entitled “Mobile station handover method for asynchronous wireless telecommunication system, involves switching between USTS and non-USTS modes, based on ratio of power intensities of pilot signals of current and adjacent cells” by J. Cho et al. describes a method of switching between an uplink synchronous transmission scheme (USTS) and a non-USTS mode based on the ratio of power intensities between a current cell and adjacent cell. The method is used for handover of a mobile station in a wireless telecommunication system and to increase data transmission rate based on the compressed mode. The method described by Cho et al. is not directed to a cell search and the ratio of pilot power intensities that are described and used by Cho et al. vary from cell to cell. The ratio of power intensities can be determined based on a snapshot of the correlation data (an instant of time).